A conventional carburetor control system is shown in Japanese Patent Application Laid-open Nos.91356/82 and 77065/84, in which the amount of bleed air flowing through the main bleed air control passage and the slow air control passage is controlled by a solenoid valve to set the air-fuel ratio in the internal combustion engine to a value corresponding to the operational condition of the engine. However, this prior art system requires a fuel whose calorific value is substantially constant. Therefore, when operating the internal combustion engine using fuels having calorific values outside of the range, it is difficult to control the air-fuel ratio in conformity with the operational condition of the engine.
The present invention has been accomplished with such circumstance in view, and it is an object of the present invention to provide an air-fuel ratio control system for an internal combustion engine, wherein a plurality of types of fuel with different calorific values can be used, and the bleed air control quantity can be varied finely and stably to follow a variation in calorific value, thereby enabling a fine control of the air-fuel ratio.